U.S. patent number 4,531,046 [Application Number 06/456,735] was granted by the patent office on 1985-07-23 for beverage brewing apparatus with constant temperature water reservoir.
This patent grant is currently assigned to Bunn-O-Matic Corporation. Invention is credited to Kenneth W. Stover.
United States Patent |
4,531,046 |
Stover |
July 23, 1985 |
Beverage brewing apparatus with constant temperature water
reservoir
Abstract
A beverage brewing apparatus includes a hot water reservoir from
which heated water is dispensed in predetermined serving volumes
through a beverage filter to a serving beaker. A volume of cold
water equal to a predetermined serving volume is admitted to a
lower inlet zone of the reservoir, heated by an electric resistance
heating element within the reservoir and dispensed in a volume
equal to the serving volume from an upper outlet zone of the
reservoir coincident with the introduction of cold water into the
inlet zone. The reservoir has a volume greater than twice the
predetermined serving volume so that a volume of resident heated
water is contained in the central brew zone thereof. A temperature
sensing element, e.g., thermistor, is disposed within the lumen of
a hollow heat conductive tubing extending to a predetermined
location in the brew zone at or near the center of the reservoir
and produces a temperature control signal continuously indicative
only of the temperature of the water in the brew zone. An
electronic control circuit varies the duty cycle of the heating
element over recurring timing intervals in response to the control
signal to maintain the water in the brew zone at a predetermined
temperature. The inlet and outlet zones correspond to the lower
one-fourth and the upper one-fourth volumes, respectively, of the
reservoir.
Inventors: |
Stover; Kenneth W.
(Springfield, IL) |
Assignee: |
Bunn-O-Matic Corporation
(Springfield, IL)
|
Family
ID: |
23813936 |
Appl.
No.: |
06/456,735 |
Filed: |
January 10, 1983 |
Current U.S.
Class: |
392/442; 392/447;
99/281; 99/288; 99/305; 99/307 |
Current CPC
Class: |
A47J
31/56 (20130101) |
Current International
Class: |
A47J
31/44 (20060101); A47J 31/56 (20060101); H05B
001/02 (); F24H 001/20 (); A47J 031/057 () |
Field of
Search: |
;219/296-299,306-308,283,331,328,523
;99/279-282,288,305,306,304,307 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Bartis; A.
Attorney, Agent or Firm: Lockwood, Alex, Fitzgibbon &
Cummings
Claims
I claim:
1. A beverage brewing apparatus comprising:
a hot water reservoir of predetermined volume;
means including a resistance heating element within said reservoir
operable from an applied electric current for heating water in the
reservoir;
inlet means for admitting a volume of cold water equal to a
predetermined serving volume into an inlet zone at the bottom of
said reservoir;
outlet means for discharging a volume of heated water equal to said
predetermined serving volume from an outlet zone at the top of said
reservoir coincident with the introduction of water by said inlet
means into said reservoir;
said reservoir having a volume substantially greater than twice
said predetermined serving volume whereby a volume of resident
heated water is contained within a central brew zone thereof;
a segment of hollow heat-conductive tubing extending from a
location at one end exterior to the reservoir to a predetermined
location in said brew zone of said reservoir;
temperature sensing means disposed within the lumen of said tubing
segment in thermal communication with said tubing segment at said
predetermined location for producing a temperature control signal
continuously indicative only of the temperature of the water at
said predetermined location in said brew zone, and substantially
non-indicative to the temperature of said water in said inlet and
outlet zones; and
control circuit means responsive to said control signal for varying
the duty cycle of the heating element over recurring timing
intervals to maintain said water in said brew zone at a
substantially constant predetermined temperature.
2. A beverage brewing apparatus as defined in claim 1 wherein said
inlet zone corresponds to approximately the bottom 1/4 volume of
said reservoir, and said outlet zone corresponds to approximately
the top 1/4 volume of said reservoir.
3. A beverage brewing apparatus as defined in claim 1 wherein said
inlet means include a tube extending vertically in said reservoir
for conveying cold water to said inlet zone.
4. A beverage brewing apparatus as defined in claim 1 wherein said
temperature sensing means comprise a thermistor.
Description
BACKGROUND OF THE INVENTION
The present invention is directed generally to brewing apparatus
for heated beverages, such as coffee and tea, and more particularly
to brewing apparatus wherein a supply of hot water for producing
the beverage is maintained at a predetermined constant brewing
temperature within a heated reservoir by circuitry continuously
responsive to the temperature of the contents.
One known type of brewing apparatus for making heated beverages
includes a reservoir within which a volume of water to be displaced
is heated by a resistance heating element to a predetermined
brewing temperature. In a preferred form of such brewing apparatus,
such as the coffee maker described in U.S. Pat. No. 4,413,552 to
Donald L. Daugherty, heated water is displaced from the top
portion, or outlet zone, of the reservoir by cool or cold water
entering the bottom portion, or inlet zone, and discharged onto
ground coffee or tea held in a brewer funnel lined with a
disposable filter. The freshly brewed coffee or tea discharging
from the brewer funnel is collected in a serving beaker.
Cold water is admitted in batches of predetermined volumes to the
reservoir of such brewing apparatus to displace the heated water
delivered to the brewing funnel. In pour-in type beverage brewers,
such as described in the afore-identified U.S. Pat. No. 4,413,552,
a quantity of cold water sufficient to produce the desired volume
of beverage to be brewed is poured into a cold water basin from
which it flows by gravity into a hot water tank to displace an
equal quantity of hot water to the brewing funnel. In automatic
type beverage brewers, such as described in U.S. Pat. No.
3,793,934, a valve is opened by electrical or manual means to
periodically deliver the batches of cool or cold water to the
apparatus.
Prior art beverage brewers, such as described in U.S. Pat. No.
3,736,155, have employed a bimetallic switch which energized the
resistance heating element when the water temperature in the
reservoir fell to a predetermined minimum below the brewing
temperature, and de-energized the heating element when the water
temperature in the reservoir rose to a predetermined maximum above
the brewing temperature. The difference between such minimum and
maximum temperatures was typically from 6 to 8 degrees Fahrenheit.
In this way, water in the hot water tank was cycled between minimum
and maximum temperatures, through an average temperature
corresponding to the desired brewing temperature. Thermostat
switches responsive to temperatures sensed by bulb and capillary
type temperature sensing elements disposed in the hot water tanks
have also been used. To minimize the frequency of such cycling
between minimum and maximum hot water temperatures, an auxiliary
heat source in the form of a continuously-excited resistance
heating blanket was wrapped around the reservoir to provide a
continuous source of heat for the water contained within the
reservoir.
For maximum operating efficiency and minimum scale formation within
the reservoir, it is desirable that the spread between minimum and
maximum hot water temperatures be minimized and preferably
substantially eliminated. The present invention is directed to an
improved beverage maker of the type maintaining a supply of hot
water wherein the heating element in the hot water reservoir is
periodically excited over a variable duty cycle to maintain a
substantially constant water temperature in the reservoir, thereby
minimizing scaling, maximizing operating efficiency and eliminating
the need for a heated jacket.
Accordingly, it is a general object of the present invention to
provide a new and improved beverage brewer.
It is a more specific object of the present invention to provide a
new and improved beverage brewer having improved operating
efficiency and reduced tendency for scale formation.
It is a further object of the present invention to provide a new
and improved beverage brewer of the type having an internal
reservoir for maintaining a supply of heated water wherein the
temperature of the water is maintained at a substantially constant
predetermined brewing temperature.
Still another object of the invention is the provision of a new and
improved beverage brewer of the type housing a hot water tank from
which hot water is displaced to a brewing funnel and wherein the
water temperature is maintained substantially constant so as to
eliminate the need to provide the tank with a heated jacket and
thereby avoiding the detrimental effects of such blankets,
manifested in corrosion and cracking of the tank wall.
It is a still further object of the present invention to provide a
new and improved beverage brewer of the type having an internal hot
water reservoir wherein a heating element in the reservoir is
periodically excited over a variable duty cycle to maintain a
substantially constant predetermined brewing temperature.
SUMMARY OF THE INVENTION
A beverage brewing apparatus includes a hot water reservoir of
predetermined volume, and means including a resistance heating
element within the reservoir operable from an applied electric
current for heating water in the reservoir. Inlet means admit a
volume of cold water equal to a predetermined serving volume into
an inlet zone at the bottom of the reservoir, and outlet means
discharge a volume of heated water equal to the predetermined
serving volume from an outlet zone at the top of the reservoir
coincident with the introduction of water by the inlet means into
the reservoir. The reservoir has a volume substantially greater
than twice the predetermined serving volume whereby a volume of
resident heated water is contained within a central brew zone
thereof. A segment of hollow heat-conductive tubing extends from a
location exterior to the reservoir to a predetermined location in
the brew zone. Temperature sensing means disposed within the lumen
of the tubing segment in thermal communication with the tubing
segment at the predetermined location produce a temperature control
signal continuously indicative only of the temperature of the water
in the brew zone, and substantially non-indicative of the
temperature of the water in the inlet and outlet zones, and control
circuit means responsive to this control signal vary the duty cycle
of the heating element over recurring timing intervals to maintain
the water in the brew zone at a substantially constant
predetermined temperature.
BRIEF DESCRIPTION OF THE INVENTION
The features of the present invention which are believed to be
novel are set forth with particularity in the appended claims. The
invention, together with the further objects and advantages
thereof, may best be understood by reference to the following
description taken in conjunction with the accompanying drawings, in
the several figures of which like reference numerals identify like
elements, and in which:
FIG. 1 is a perspective view of a cold water pour-in type coffee
brewer forming one embodiment of the present invention shown in
conjunction with three coffee beakers and a removable brewer funnel
in broken outline.
FIG. 2 is a vertical sectional view of the coffee brewer of FIG. 1,
certain parts therein being shown in elevation for clarity.
FIG. 3 is a partial top plan view of the coffee brewer taken along
line 3--3 of FIG. 2.
FIG. 4 is a simplified perspective view of certain principal
elements of the coffee brewer and the electrical circuitry
associated therewith.
FIG. 5 is a simplified schematic diagram partially in functional
block form of the resistance heater excitation control circuit of
the coffee brewer.
FIG. 6 is a depiction of reservoir water temperature as a function
of time useful in understanding the operation of the coffee
brewer.
FIG. 7 is a depiction of certain waveforms useful in understanding
the operation of the brewer.
FIG. 8 is a depiction of certain waveforms useful in understanding
the operation of the heater control circuit of the coffee
brewer.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the figures, and particularly to FIG. 1, a cold water
pour-in coffee maker 10 incorporating the invention is seen to have
a generally C-shaped body 11 which includes an upper body portion
12, a lower body portion 13, and an interconnecting upright body
portion 14. The coffee maker body 11 is fabricated in a
conventional manner, preferably from stainless steel sheet, but may
be fabricated from other metals or from known plastics having
suitable strength and durability. A brewer funnel 15 of
conventional construction is removably supported in a conventional
manner by guide rails 16 underneath the upper body portion 12. A
coffee serving beaker 17 is removably supported on the bottom body
portion 13 underneath the brewer funnel 15 on a heated warming
plate 18 mounted on the top surface of the lower body portion
13.
A cold water pour-in opening 20 (FIG. 2) is located on the top
front portion of the upper body portion 12. Rearwardly thereof, a
top warmer unit 21 is mounted which includes a pair of warming
plates 22 and 23 on which two additional serving beakers 24 and 25
may be mounted.
The upper body portion 12 and the central vertical body portion 14
house an integral, inverted L-shaped combination elongated cold
water basin and hot water reservoir assembly indicated generally at
30 in FIGS. 2-4. The elongated cold water basin 31 of the assembly
is preferably die-pressed in a conventional manner from stainless
steel sheet so as to have a continuous seamless bottom of
progressively increasing depth toward the rear of the coffee
brewer. The hot water reservoir 32 of the assembly is welded to a
bottom opening 33 (FIG. 2) formed adjacent the deep end of the
basin 31. The reservoir 32 is also preferably fabricated from
stainless steel sheet in a conventional manner. As best seen in
FIG. 2, the bottom of the basin 31 slopes toward recess 33 so that
cold water poured into basin 31 will flow and drain into reservoir
32.
In a coffee brewer capable of making two quart batches of coffee,
the hot water reservoir 32 may have, by way of illustrative
example, an inside diameter of 275 cubic inches (i.e. slightly over
two fluid quarts). The capacity of the basin 31 for use with a hot
water reservoir 32 of such capacity may, for example, be 155 cubic
inches with the interior of the basin having a length of 151/4
inches, a width of 61/4 inches and a depth ranging from 13/4 inches
to 11/2 inches.
The circular opening communicating between the bottom of basin 31
and the top of hot water reservoir 32 is closed by a disk-shaped
baffle 34. A central opening 35 is provided in the disk 34 which
communicates with the upper end of a vertical cold water tube 36,
the upper end of which is welded or otherwise suitably joined to
the underside of disk 34. The bottom end of the cold water tube 36
terminates adjacent the bottom of reservoir 32 so that as cold
water flows downwardly through tube 36 it is discharged in the
lower portion of reservoir 32, thereby displacing upwardly the hot
water contained in the reservoir. This manner of operation is well
known in connection with coffee brewers of the cold water pour-in
type.
An inverted siphon cup 40 (FIG. 2) is positioned adjacent the top
of reservoir 32 near the removable brewer funnel 15. The siphon cup
40 is carried by the inner end of a nipple 41 which projects in
fluid-tight relationship through an aperture in the wall of
reservoir 32. The outer end of the nipple 41 connects with a
downwardly slanted tube 42 having a down-turned outer or distal end
43 on the lower end of which a hot water spray head 44 is fastened
in a conventional manner.
Water within reservoir 32 is heated in a conventional manner by a
heating element 45, which may take the form of a Calrod resistance
heating element or other known electrically energized heating
element. The top of basin 31 is covered and enclosed by a cover
member 46 (FIG. 2) which forms the top of the upper body portion
12. The cold water pour-in opening 20 is formed in the cover 46
adjacent the front of the coffee maker and preferably is provided
with a screen 47 so as to prevent foreign objects from entering the
enclosed basin 31. The cover 46 completely encloses the cold water
basin 31 and serves as a floor support for the top heater unit
21.
Referring to FIG. 4, electrical power is supplied to three
resistance heating elements 50, 51 and 52 associated with hot plate
assemblies 18, 22 and 23, respectively, and to the resistance
heating unit 45 contained within reservoir 32, by electrical
circuitry within housing 11. Electrical power is supplied to this
circuitry by a conventional line cord 53. A first electrical switch
54 on the front surface of housing portion 12 controls the
application of power to the two resistance heater elements 51 and
52 associated with the top hot plates 22 and 23. A second
electrical switch 55 controls the application of electrical power
to resistance heating element 50 associated with the bottom hot
plate 18. A third electrical switch 56 controls the application of
electrical power to a control circuit 57, which in turn controls
electrical excitation of the resistance heating element 45 in hot
water reservoir 32. Visual indication of the operation of the
various resistance heaters is provided by neon-type indicator lamps
58, 59 and 60, which may be incorporated integrally within switches
54-56 to illuminate upon actuation of the respective circuits.
One side of the A/C line is connected to one terminal of each of
switches 54-56. Upon closure of switch 54, power is applied to
resistance heating elements 51 and 52 and indicator lamp 58. This
causes the hot plates 22 and 23 to heat, and coffee placed on these
plates as in beakers 24 and 25 to be kept warm for serving. Upon
closure of switch 55, resistance heating element 50 and indicator
lamp 59 are energized. This causes the bottom hot plate 18 to heat
coffee contained in serving beaker 16. Closure of switch 56 causes
control circuit 57 to be energized. This control circuit works in
conjunction with a temperature probe 61 within reservoir 32 to
control the excitation of heating element 45 to heat water in the
reservoir to a uniform predetermined serving temperature.
As best seen in FIG. 2, the temperature probe 61 consists of a
semi-flexible tubing segment 62 which extends through cover 35
downwardly into and near the generally central portion of reservoir
32. An upper portion of this tubing segment 62 is directed at a
generally right angle through the rear side wall of basin 31
through an aperture provided for that purpose. The tubing segment
62, which is preferably formed of a heat-conductive metal such as
copper, is closed at its bottom end and extends through plate 34
and the rear wall of basin 31. To sense temperature within the
reservoir, a thermistor 63 (FIG. 2) is positioned near the tubing
end. A pair of electrical conductors 64 and 65 extend from the
thermistor through tubing segment 62 to control circuit 57.
The tubing segment 62 affords protection to thermistor 63 against
exposure to liquid within reservoir 32 or basin 31. In addition,
the semi-rigidity of tubing segment 62 allows thermistor 63 to be
accurately positioned near the central portion of reservoir 32.
Adjustment of the exact position may be conveniently accomplished
by manually deforming the tubing segment 62 until the thermistor 63
has been properly positioned. In practice, where cold water is
admitted to the bottom 1/4 portion, or inlet zone, of the
reservoir, and heated water is withdrawn from the top 1/4 portion,
or outlet zone, of the reservoir, it has been found that thermistor
63 is preferably disposed at or near the center, or brew zone, of
the reservoir so as to optimize its response to water changes.
A temperature adjustment control 66 (FIG. 2) may be provided in
conjunction with control circuit 57 to enable the user to select a
desired brewing temperature at which the water within reservoir 32
is to be maintained. A pair of grommets 67 may be provided to
protect the tubing segment 67 from possible damage as a result of
direct contact with cover plate 34 or the rear wall of basin 31 as
it passes through the apertures provided in these elements. A
plurality of retaining clips 68 secured to the bottom of basin 31
by screws 69 or other appropriate means may be provided to secure
cover 34 in position and prevent inadvertent repositioning of
thermistor 63 within hot water reservoir 32.
The operation of coffee brewer 10 will be readily understood since
it generally parallels the operation of known cold water pour-in
coffee brewers. In putting brewer 10 into operation, sufficient
cold water is poured into basin 31 so as to completely fill hot
water reservoir 32. The fact that reservoir 32 is filled will be
known when water commences to flow out through the side tube 42 and
discharge through the spray head 44. When the hot water reservoir
is filled, heating element 45 is energized and, thereafter, heating
elements 50, 51 and 52 are energized as needed. Once the cold water
contents of reservoir 32 have had an opportunity to come to the
desired serving temperature, a filter with the proper amount of
ground coffee (or tea) may be placed in the brewer funnel 15 and
inserted into place on the underside of the top body portion 12 so
as to be supported beneath spray head 44 on the rails 16 mounted on
the underside of body portion 12.
Assuming that a two quart batch of coffee is to be brewed, a
pitcher containing two quarts of cold water is dumped into basin 31
through opening 20. The cold water flows downwardly through cold
water tube 36 at a sufficiently rapid rate so that basin 31 will
not overflow, even with fast dumping of the two quarts of cold
water through the opening. The entrance of the cold water into the
bottom portion of the hot water reservoir 32 is at such a
restrained rate that it does not mix to a substantial extent with
the hot water in the reservoir, but rather, the incoming cold water
displaces the hot water upwardly so that it flows out through side
tube 42. Once this flow has started, it continues due to siphoning
action until the water level in reservoir 32 drops below and
exposes the bottom of the inverted siphon cup 40. In a manner well
known to the art, hot water sprays from the spray head 44 onto the
ground coffee (or tea) in the brewing funnel 15, and the coffee
beverage forms in the funnel and discharges through the bottom
opening of funnel 15 into serving beaker 17.
It will be appreciated that up to three batches or beakers of
coffee may be prepared and maintained at one time on coffee brewer
10 by using the three hot plates 18, 22 and 23. It will also be
appreciated that the interior of the hot water reservoir 32 can be
readily reached for cleaning by simply removing cover 46 and
lifting the cover together with heater unit 21 and temperature
probe 61 from the upper body portion 12. Since periodic cleaning or
servicing of the interior of the reservoir is normally required,
this ready access is a highly desirable feature. Furthermore, the
inverted L-shaped configuration of the cold water basin 31 and the
hot water reservoir 32 allows the coffee brewer 10 to have a
relatively narrow profile from left to right so that it takes up
only a small space from one side to the other on a countertop or
table. Generally, counter space is at a premium and often limited
or restricted, so that this narrow configuration is highly
desirable.
Coffee brewer 10 includes, in accordance with the invention, a
novel system for maintaining water in reservoir 32 at a preselected
temperature for brewing. As shown in FIG. 4, this system includes
thermistor 63, heating element 45, and control circuit 57. As
previously described, control circuit 57 functions in response to a
control signal developed by thermistor 63 to vary the electrical
excitation of heating element 45 so as to maintain a uniform water
temperature.
Referring to FIG. 5, the control circuit 57 may advantageously be
constructed as a zero voltage switch wherein the number of mains
cycles applied to heating element 45 in a predetermined fixed
timing period is varied as a function of the temperature sensed by
thermistor 63. To this end, and with reference to FIG. 5, the
alternating A/C mains current from power switch 56 is applied to a
diode 70 which serves as a source of direct current for the control
circuit. Direct current is applied to a regulator stage 71 which
provides a constant voltage source for application over conductor
64 to thermistor 63. The other terminal of thermistor 63 is
connected through conductor 65 to one input of a comparator
amplifier 72, and to ground through the series combination of the
temperature control potentiometer 66, a trim potentiometer 73 and a
fixed resistor 74. With this arrangement, thermistor 63,
temperature control potentiometer 66, trim potentiometer 73 and
resistor 74 form a voltage divider, and the voltage applied to
comparator amplifier 72 is a function of the temperature of
thermistor 63. The other input of comparator amplifier 72 is
connected to the output of a ramp generator 75, which functions to
repetitively produce ramp signals of progressively increasing
voltage and fixed D/C offset during fixed time intervals of a slope
determined by a timing circuit comprising a resistor 76 and
capacitor 77 connected between the output of diode 70 and system
ground. A capacitor 78 connected between diode 70 and system ground
filters the output of the diode to provide a filtered direct
current for operation of the system.
Operation of ramp generator 75 is initiated at the beginning of
each timing period by a period pulse generator 80 which functions
in response to the applied alternating current to produce periodic
pulses which initiate a periodic timing period T utilized in the
operation of the system. These pulses are applied to ramp generator
75, wherein they initiate the generation of a ramp function signal,
and to a latch circuit 81 which resets upon receipt of each pulse.
The latch circuit 81 remains in the reset state until an output is
applied from comparator 72 upon a comparison being made between the
thermistor control signal and the ramp signal, at which time the
latch actuates to a set state.
The output of latch 81 is supplied to one input of an AND gate 82.
The remaining input of AND gate 82 received pulses from a delayed
pulse generator 83. Pulse generator 83 is keyed by a zero voltage
crossing detector 84, so as to produce an output pulse after a
predetermined short delay following each zero crossing of the
applied alternating current waveform. The degree of delay between
the production of an output pulse by pulse generator 83 and the
zero crossing of the applied alternating current waveform is
determined by a timing capacitor 85 associated with pulse generator
83.
When AND gate 82 is enabled by latch 81, the delayed pulses
produced by generator 83 at the beginning of each half cycle are
applied to the gate electrode of a triac 86 having principal
electrodes connected between resistance heating element 45 and the
reference side of the A/C line. Thus connected, triac 86 responds
to each of the applied pulses to conduct during the succeeding half
cycle of the applied alternating current waveform, thereby
energizing heating element 45 in a manner well known to the
art.
Since AND gate 82 is enabled only in the event that a comparison
has not been detected between the inputs of comparator amplifier
72, the duty cycle of heating element 45 is dependent on the
temperature sensed by thermistor 63. When the temperature of the
water in reservoir 32 is less than the predetermined desired
serving temperature, as when the water is being initially heated,
the voltage division effected by resistors 66, 73 and 74 in
combination with thermistor 63 does not reach the offset level of
the ramp signal and heating element 45 is continuously excited.
However, once the operating temperature has been attained, and the
voltage applied to comparator amplifier 72 becomes equal to that
applied by ramp generator 75 at some point during a timing period,
latch 81 switches to a set state, gate 82 is inhibited, and gating
pulses are not applied to triac 86. As a result, heating element 45
is not energized for the duration of the timing period. Upon
initiation of the succeeding timing period, latch 81 is again
conditioned to a reset state and comparator amplifier 72 does not
condition a set state until a comparison is again made, at which
time the application of half-cycle gating pulses to triac 86 is
again terminated for the balance of the succeeding timing
period.
The timing period T is sufficiently long so that multiple cycles of
the applied A/C mains current are available to the heating element
during any one timing period. Thus, during each timing period
heating element 45 is energized for a number of complete
half-cycles dependent on the temperature of the water. If the water
is cooler than desired, a greater number of half-cycles is applied.
If the water is hotter than desired, a lesser number of half-cycles
is applied.
The mode of operation of coffee brewer 10 is further illustrated by
the waveforms of FIGS. 6 and 7. As shown in FIG. 6, upon initial
power-up of coffee brewer 10, the temperature of the water in
reservoir 32, as illustrated by plot 90, rises with time until the
preselected serving temperature T.sub.nom is reached, at which time
the temperature rise levels off. This is in contrast to prior art
thermostats wherein the temperature fluctuated between a maximum
temperature T.sub.max and a minimum temperature T.sub.min, as shown
by the plot 91.
The manner in which the leveling off of the temperature is achieved
as illustrated in FIG. 7, wherein the voltage at comparator 72 as
applied by thermistor 63 is plotted against time in conjunction
with the ramp function applied to the comparator by ramp generator
75. If the ramp function is taken as varying between an initial
offset voltage V.sub.0 and a final voltage V.sub.f, it is seen that
the voltage V.sub.t from thermistor 63 eventually rises, as
depicted by plot 92, to a level greater than V.sub.0. Up to that
point, the excitation applied to heating element 45 is continuous
and maximum heating of the water in reservoir 32 is achieved.
However, upon the thermistor voltage exceeding V.sub.0, the
excitation applied to the heating element is reduced in proportion
to the extent that the voltage V.sub.t exceeds V.sub.0, thereby
causing the temperature of the water in the reservoir to stabilize
at some selected temperature which will provide a voltage V.sub.t
intermediate the minimum and maximum voltages V.sub.0 and V.sub.f
of the ramp function.
This is illustrated in greater detail in FIG. 8. Here, the
thermistor output voltage V.sub.t is seen to be situated at a
voltage level approximately intermediate the minimum and maximum
voltage levels of the ramp function 93, such that the voltage
comparison is achieved at a location generally intermediate the
timing period T established by period pulse generator 80. The ramp
function 93 is seen to have a repetition rate identical to the
timing period T. The thermistor output voltage plot 94 is seen to
lie intermediate the high and low voltage limits of the ramp
function, a voltage crossing point 95 being established within the
timing period T.
Upon initiation of each timing period T, the period pulse generator
80 produces a period pulse, as shown by waveform 96. This timing
pulse initiates the formation of a ramp function by ramp generator
75 and conditions latch 81 to a set state. Consequently, AND gate
82 is enabled and keying pulses developed by pulse generator 83
following each zero crossing of the applied alternating current
line voltage detected by crossing detector 84 are applied to the
gate electrode of triac 86. These firing pulses, as depicted by
waveform 97, continue until a comparison is achieved between the
ramp function 93 and the thermistor voltage 94, at which time latch
81 is conditioned to a reset state, AND gate 82 is inhibited, and
triac 86 becomes nonconductive. Consequently, the alternating
current applied to heating element 45, as depicted by waveform 98,
is interrupted for the remaining portion of the timing cycle T,
shown as T.sub.off.
It will be noted that only complete half-cycles of the applied A/C
mains current, as depicted by waveform 99, are applied to heating
element 45 while triac 86 is conductive. This minimizes the
electrical interference often associated with switching resistance
heating devices during periods of current flow, as when using
bimetallic-type thermostat switches, and avoids introducing a D/C
component into the A/C mains supply from uneven half-cycle current
demands.
Should the temperature of the water in reservoir 32 increase, a
comparison is achieved earlier between the ramp function 93 and the
increased voltage of the thermistor, as depicted by waveform 94a,
resulting in a crossing point 95a earlier in the timing period.
Consequently, the heating element 45 will be energized for a lesser
portion of the timing cycle, tending to cool the water within the
reservoir. Conversely, should the temperature of the water fall,
resulting in a reduced voltage V.sub.t, as depicted by waveform
94b, then the comparison with the ramp function 93 will occur at a
point 95b later in the timing cycle, resulting in the heating
element 45 being energized for a greater portion of the timing
cycle, thereby tending to raise the temperature of the water with
the reservoir.
In a commercial version of the coffee brewer having the capacity
and dimensions previously described and operable from 120 VAC 60
cycle current, the timing period T may be 2.0 seconds, and the
brewer may be operable over a temperature range of 165.degree. F.
to 210.degree. F., depending on the setting of potentiometer
66.
The components contained within the dotted enclosure 87 may be
conveniently incorporated within a single integrated circuit. One
such integrated circuit which has proven successful in this
application is the Model SL441A Zero Voltage Switch manufactured by
Plessey Semiconductors. Other integrated circuits, such as the
Motorola Model UAA1016 may be utilized instead.
Thus, the function of control circuit 57 is to maintain the water
within reservoir 32 at a predetermined brewing temperature for
brewing intermediate maximum and minimum temperatures corresponding
to maximum and minimum voltages developed by the ramp generator 75.
Should the temperature fall below this temperature range, as during
initial power-up or following the brewing of a large quantity of
beverage, the heating element will be continuously energized during
the entire duration of each timing period and the water temperature
will be raised as quickly as possible to the selected nominal
serving temperature.
Control circuit 57 (FIG. 4) provides continuous control of the
excitation applied to heating element 45, resulting in
significantly improved control of water temperature within the
water reservoir 32. This obviates the need for auxiliary heating
elements, such as resistance heating blankets utilized in
conjunction with prior art thermostat devices, and assists in
reducing scaling by subjecting the heating element to continuous
short-term cycling within each timing period.
Although the invention has been shown in conjunction with a pour-in
type beverage brewer, it will be appreciated that the invention can
be practiced in other beverage brewers having heated water
reservoirs, including automatic-filling brewers wherein cold water
is admitted to the reservoir through a valve, which may be actuated
by means of a timing circuit or other known type of measuring
system.
While a particular embodiment of the invention has been shown and
described, it will be obvious to those skilled in the art that
changes and modifications may be made therein without departing
from the invention in its broader aspects, and, therefore, the aim
in the appended claims is to cover all such changes and
modifications as fall within the true spirit and scope of the
invention.
* * * * *